1. Field of the Invention
The present invention relates to an ultrasonic apparatus and an ultrasonic diagnostic method for obtaining biological information inside an object through a transmission and a reception of an ultrasonic wave to and from the object, particularly to the ultrasonic apparatus and the ultrasonic diagnostic method capable of measuring or using a frequency dependent-attenuation affecting a ultrasonic wave in a living body.
2. Description of the Related Art
An ultrasonic apparatus is a diagnostic imaging apparatus which transmits and receives the ultrasonic wave to and from the object, such as the living body, to non-invasively obtain a tomographic image of a tissue present in the object. It is known that, when the ultrasonic apparatus transmits the ultrasonic wave in the form of plane wave to the object, the ultrasonic wave is affected by a frequency dependent attenuation (FDA) along with a propagation of the ultrasonic wave. The amount of the FDA is determined depending on the frequency dependent-attenuation coefficient β. For example, when an ultrasonic pulse having a center frequency f0 of 2.5 [MHz] is transmitted to the object with a frequency dependent-attenuation coefficient β=1 [dB/MHz/cm] to obtain an ultrasonic reflection echo of a matter located at a depth of z=10 [cm] in the object, the ultrasonic pulse is affected by attenuation “At” expressed by a following equation (1).At=2β·f0·z=1[dB/MHz/cm]×2.5[MHz]×10[cm]×2[a round-trip]=50 dB  (1)
To measure the frequency dependent-attenuation coefficient β of the living tissue by using a frequency analysis such as a fast Fourier transformation (FFT), at least a predetermined number of data sets in a target area are required. However, the value of the frequency dependent-attenuation coefficient β is not necessarily the same even within the same object, and varies depending on an organ and pathology. Thus, it is difficult to secure a sufficient number of data sets, and the value of the frequency dependent-attenuation coefficient β in the object is unknown. A public known ultrasonic diagnostic apparatus uses a broadband pulse wave as the ultrasonic pulse to be transmitted. Therefore, the deeper source of the ultrasonic reflection echo is located, the more the ultrasonic pulse is affected by the FDA and reduced in the center frequency.
On the other hand, to obtain a received signal received as the ultrasonic reflection echo with a good S/N (signal to noise ratio), it is important to adjust a mixing frequency used in a quadrature phase detection to the center frequency of the received signal in accordance with the depth. However, the value of the frequency dependent-attenuation coefficient β of the object constituting the living body is unknown. Usually, therefore, a site for imaging is assumed, and the mixing frequency is changed in accordance with the depth and in consideration of an average frequency dependent-attenuation coefficient β of the site. For example, a method of determining the value of the mixing frequency through the frequency analysis of the received signal has been proposed (e.g., Japanese Patent Application Publication (Laid-open: KOKAI) No. 2003-235844).
In the imaging in a color Doppler mode for displaying information of a blood flow speed according to an ultrasonic Doppler method, the speed “v” of the blood flow is calculated from a following equation (2) using a normalized frequency “fd” of a detected ultrasonic Doppler signal, the center frequency “fm” of the received signal, a speed of sonic “C”, and a pulse repetition frequency “PRF”. The normalized frequency “fd” ranges from −0.5 to 0.5.
                    v        =                                                            C                ·                P                            ⁢                                                          ⁢              R              ⁢                                                          ⁢              F                                      2              ⁢                                                          ⁢                              f                m                                              ⁢                      f            d                                              (        2        )            
To measure the distortion of the tissue by using a tissue Doppler method, more accurate information of the blood flow speed is required. Thus, it is desired to measure and correct the center frequency of the received signal. In view of this, a method of measuring and correcting the center frequency of the received signal through the frequency analysis of the received signal has been proposed (e.g., Japanese Patent Application Publication (Laid-open: KOKAI) No. 2005-58533).
On the other hand, a technique of visualizing the frequency dependent-attenuation coefficient β such that the frequency dependent-attenuation coefficient β directly contributes to the diagnosis has been proposed (e.g., Japanese Examined Patent Application Publication No. 3-60493). Specifically, there is a method of calculating the frequency dependent-attenuation coefficient β through the frequency analysis of the received signal and visualizing the calculated frequency dependent-attenuation coefficient β. Further, a method of measuring in real time the frequency dependent-attenuation coefficient β by using a spectral moment method has been proposed (e.g., Japanese Examined Patent Application Publication (Laid-open: KOKOKU) No. 5-41259).
That is, the frequency dependent-attenuation coefficient β of the tissue has been conventionally measured by using the frequency analysis. The method of the frequency analysis includes a method of analysis based on a frequency axis, such as a Fourier transformation, and a method of analysis based on a time axis, such as the spectral moment method.
However, the sites in the living body rarely have a uniform impedance difference. Therefore, even if the frequency analysis is performed in a certain range to measure the frequency dependent-attenuation coefficient β, the range includes a scatterer having a small impedance difference, such as a parenchyma of the liver, and a reflector having a large impedance difference, such as a blood vessel wall and a tissue boundary. Further, a ratio of the scatterer having the small impedance difference or the reflector having the large impedance difference is different from site to site in the living body.
Accordingly, if a frequency characteristic is compared between sites of different conditions, an accurate frequency dependent-attenuation coefficient β cannot be measured. Particularly, an error is large in the intensity of the ultrasonic reflection echo required for the calculation of attenuation.
In addition, there is a circumstance in which a processing of the frequency analysis is generally complicated.
Further, an imaging according to a contrast echo method using an intravenous ultrasonic contrast agent has been recently performed. It is desired in the contrast echo method to diagnose the hemodynamics on the basis of the comparison of a degree of the contrast produced by the contrast agent among the imaged sites. As described above, however, the frequency dependent-attenuation coefficient β varies depending on the depth and the organ. Thus, a circumstance arises in which simple comparison of a luminance of the contrasted ultrasonic image cannot be performed among the imaged sites. Therefore, it is desired to provide ultrasonic image information enabling the simple comparison of the luminance, for example, irrespective of the differences in the frequency dependent-attenuation coefficient β, and thus more useful for the diagnosis.